I write about the science of food and cooking: where our foods come from, what they are and what they're made of, and how cooking transforms them. On this site you can also find out about me, my books and my column in the New York Times, and the story of the Erice Workshops on Molecular and Physical Gastronomy. And there's a page devoted to my sister and first illustrator, Ann McGee Kurz.

It's time to spread the good word again: the Oxford Symposium invites young culinarians to apply for grants to participate in this year's edition, 6-8 July. The application deadline is 1 March.

The Oxford Symposium on Food and Cookery is the original international food conference, now in its fourth decade, and open to anyone who's interested, professional or amateur, chef or student. I attended my first in 1985. It's impressive not only for the range of subjects and participants and contributions, but also for the convivial communal meals, which nowadays are prepared by guest chefs to illuminate each year's theme. (They've come a long way since 1985!)

The Young Chefs' Grants provide the opportunity for two culinarians early in their careers to attend the Symposium, participate in its discussions, and work with the guest chefs to help prepare dinners and lunches. It's a great way to give your professional life an energizing jolt: to connect with fellow food lovers from all over the world, learn about ingredients and techniques and traditions you've never heard of, and get kitchen experience alongside some of the most knowledgeable chefs of the day.

I recently caught up with a 2015 grantee, Elizabeth Yorke, and asked her for a few words about what the Symposium experience and similar opportunities have meant to her. Elizabeth is a chef at the Red Fork Deli in Bangalore, India, and posted a scrapbook of favorite moments at the Symposium soon afterwards. Here's an excerpt of what she wrote to me last week:

When I say opportunities like the Oxford Symposium and the MAD symposium are life-changing, i'm not being overly dramatic but these events open your mind to incredible people and their ideas and this amazing thing we all share in common-- Food.

These events helped me meet fascinating people in such diverse fields. And convinced me that I need to travel every year, meet people and learn new things. Just cooking in a kitchen was not enough anymore. I met the brilliant William Rubel and interned with him studying about the histories of bread, attended a class on the theological histories of bread at Yale Divinity School, was part of the "food" themed Global Entrepreneurship Summer School in Mexico City followed by a stage at restaurant Lorea.

These small one month stints every year broadened my understanding of food and cultures and made my realise about the potential in our Indian food system and scope for research and implementation in kitchens--be it supply chains, agriculture, policy or just the simple economics of a dining plate.

Thanks, Elizabeth! Several other Young Chef grant winners have weighed in on the Symposium blog.

This year's theme is Seeds. Small word, huge subject: from porridge to nuts, biodiversity to variety patents, spices to coffee to chocolate. General-session talks will be given by scientists working on conservation and seed banks, the genomes of wheat and other crop plants, and breeding seeds specifically for breadmaking, brewing, and distilling. The smaller group discussions will be--as aways--all over the map.

Past guest chefs have included Fergus Henderson, Aglaia Kremezi, Jeremy Lee, and Allegra McEvedy. In 2018 they will be:

Olia Hercules, Ukraine-born London-based author of Kaukasis and Mamushka, with a menu from the Caucasus

Naomi Duguid, Toronto-based author of landmark books on Asia west to east, with dishes from Persia

Assaf Granit, Israeli chef-owner of Machneyuda in Jerusalem and The Palomar in London

I'm chagrined that I haven't posted anything for a full year, but glad to have a good reason to break the silence. The annual and venerable Oxford Symposium on Food & Cookery will take place this year July 8th through the 10th. And once again there are grants available to help young culinary professionals attend and work with the eminent chefs who prepare the Symposium meals.

This year's subject is

with contributions expected on a broad range of examples, from lesser cuts of meat and damaged produce to the many forms and fates of food waste, to definitions of edibility and the nature of disgust. This year's Friday night banquet, "A Bold Offal Feast," will be prepared by none other than Fergus Henderson.

Here's a summary of the Young Chefs' Grants, from the Symposium website:

The Friends of the Oxford Symposium award two fully funded places at the Symposium (8-10 July 2016) to a culinary professional or student with not more than 10 years in the industry. You will get 1 free place at the Symposium (worth £300) plus £200 cash, and be invited to work behind the scenes with our invited chef and with Tim Kelsey of the St. Catherine’s College kitchen in the preparation of Friday night’s dinner. You will then be able to attend all Symposium sessions and meals. The aim is simple: to enrich your approach to food and cooking.

To get an idea of what the experience is like, see this wonderful scrapbook summary by one of last year's grantees, Elizabeth Yorke, who attended from India. Her fellow grantee, Kiige Maagu, came from Kenya.

Last November I had the honor of being invited to join the board of Trustees of the Oxford Symposium on Food & Cookery, a now annual gathering at the University of Oxford instigated in the late 1970s by Alan Davidson and Theodore Zeldin, and taking place for the 33rd time this July 3–5. If you don't know the Symposium, here's the concise introduction from its homepage:

The Oxford Symposium is the original international conference for people with a broad interest in food, attended by scholars in different fields, enthusiastic amateurs, writers and chefs. Contributions are invited but not obligatory. The three-day meeting is a convivial event with ideas and information exchanged over great food.

There was absolutely nothing like the Symposium in its early years, when cooking had yet to become a respectable subject for scholarship of any kind. And even now that food has hit the cultural mainstream, the Symposium remains unique in its regularity and diverse mix of people, approaches, and subjects. To get a taste, watch a couple of videos of past presentations, or browse the many volumes of past Proceedings that are now freely available online here. Only the "great food" is a relatively recent development! For many years the participants were on their own for meals apart from one communal potluck, which was always interesting but, understandably given the logistical challenges, not always delicious. These days most meals are shared, and such visiting chefs as Fergus Henderson, Shaun Hill, Rowley Leigh, and Jeremy Lee participate in planning and preparing them.

I attended my first Oxford Symposium in 1985, met lifelong friends and colleagues there, and have been a fan ever since. I'm very happy to be able to work with the other Trustees to keep it going strong, and make sure that food lovers everywhere know about it and the resources it shares.

And to begin: if you are or know a young culinary professional of limited means who would like to attend this July's Symposium on "Food and Communication," you should know there's a grant that could help make it possible. There are also grants for students. Please spread the word!

I don't usually write about job openings in the food world, but a few days ago a scientist I know asked me to post a link to a help-wanted ad. I took a look and was thrilled by what I saw.

The Cornell University Department of Food Science, one of the best in the country, is hiring a new faculty member. And it wants applications not just from food scientists, but from scientists of all kinds who happen to be fascinated by food, and especially by--cooking!

Prior experience in food or ingredient science is desirable but not required if the candidate is committed to developing expertise in this area. A strong interest in culinary arts would be advantageous but is not required.

This is exciting to see, and I hope it's the beginning of a trend. It bodes well for the field of food science, and for the the growing numbers of young men and women who love both science and cooking, some of whom I hear from every year as they search for a way to combine their passions. Food science hasn't been an appealing option for many of them because its focus is primarily on manufacturing processes, safety, ingredient authentication, detailed chemical composition--but not kitchen-scale preparation, culinary traditions that have helped define excellence, the nature of deliciousness, innovation, creativity . . . . That is, not the very aspects of food that inspire passion in people.

There are understandable historical reasons for this neglect of culinary practice in mainstream food science, which has its roots in the early 20th century growth of the industry and public demand for a safe and genuine food supply. But there's no reason to continue to be limited by that history. This is the 21st century, a time of unparalleled interest in food in all its richness and complexity. How can a discipline that calls itself food science not address those aspects of food that touch people most directly? Or not tap the energy and enthusiasm of bright minds who are fascinated by what happens on their stovetops and in their crocks and carboys?

I heard about the Cornell job search from Gavin Sacks, an associate professor of enology there. I've known Gavin for several years, from research talks he's given at meetings of the American Chemical Society, and from a wonderful annual student competition for the ACS, Communicating Chemistry through cooking, which he organizes with professor Justin Miller of Hobart & William Smith Colleges. It turns out that Gavin himself came to food science indirectly. Though he ended up in enology, a very particular field of its own, I thought that his story would be interesting and encouraging to potential applicants for the new position in food science. Here's what he sent me.

My background: I got my PhD in chemistry at Cornell working on developing new instrumentation, software, and methods for use with isotope ratio mass spectrometry (IRMS). IRMS is widely used for determining the origin or authenticity of biological or geological samples, e.g. it is used in the Olympics to determine if testosterone was produced endogenously by the athlete or derived from exogenous sources; or, it can be used to determine the place of origin of a wine.

I loved the work, and I was planning on doing the conventional academic chemists’ path: finish my PhD, do a post-doc or two, apply for tenure-track positions in chemistry at appropriate schools. However, two things changed my trajectory. First, in my spare time, I got hooked on reading about the chemistry and biology of food and wine. My primary source was “On Food and Cooking” by Harold McGee, but this was supplemented by Jamie Goode’s Wine Anorak column, Ricki Carroll’s Home Cheesmaking book, and frequent trips over to the Cornell Food Science seminar series (I remember a particularly good talk by Doug Goff from University of Guelph using microscopy to study ice cream structure). I also got very interested in fermentations: towards the end of my PhD, taking a break from thesis writing meant making sauerkraut, wine, hard cider, and cheese.

The second big change was that I got married to someone with a real job in the Upstate NY area (read: not a post-doc) right after my PhD finished. I needed something to do for 6 months before my next opportunity started. I chose to work in a local winery . . . where I spent most of my time spraying Roundup during a particularly weedy summer. Although I didn’t do much winemaking, I came to realize that members of the wine and grape industry were an enthusiastic and curious group about science.

Even with these experiences, I was still planning on the “conventional” route right up until the point when I saw a job announcement by Cornell Food Science for a Wine Chemist faculty position in 2006. Even though I didn’t have a formal background in wine chemistry, my background in analysis translated well, and the other faculty were enthusiastic about helping me with the transition. It’s wonderful to be in a field where research can have a tangible effect on industry practitioners within months, and it’s also wonderful to be teaching students who love to come to class each day.

Gavin concluded with his own description of the Cornell search:

Cornell Food Science is now advertising for a new position on Ingredient Technology. While we are interested in candidates with backgrounds in food science and the food industry, we are also interested in bright candidates with knowledge that will translate to Ingredient Technology: for example, a background in macromolecular chemistry, synthetic biology, biophysics, or materials science. Food science inevitably benefits from translating advances in other fields (e.g. medicine, materials, energy) and bringing them to bear to challenging questions in food. This is an opportunity for the scientist who reads food-science related columns religiously, and who is concerned about the future of food and wants to make a difference, to take their hobby and make it into a career.

So if this sounds like you, check it out; if it sounds like someone you know, pass the word!

For me, the epitome of stovetop alchemy is making caramel from table sugar. You start with refined sucrose, pure crystalline sweetness, put it in a pan by itself, and turn on the heat. When the sugar rises above 320°F/160°C, the solid crystals begin to melt together into a colorless syrup. Then another 10 or 20 degrees above that, the syrup begins to turn brown, emits a rich, mouth-watering aroma, and adds tart and savory and bitter to its original sweetness.

That's the magic of cooking front and center: from one odorless, colorless, simply sweet molecule, heat creates hundreds of
different molecules, some aromatic and some tasty and some colored.

How does heat turn sugar into caramel? Heat is a kind of energy that makes atoms and molecules move faster. In room-temperature table sugar, the sucrose molecules are jittery but standing in place, held still by the forces of attraction to their neighbors. As the sugar heats up in the pan, its molecules get more and more jittery, to the point that their jitters overcome the attractive forces and they can jump from one set of neighbors to another. The solid crystals thus become a free-flowing liquid. Then, as the temperature of the sugar molecules continues to rise, the force of their jittering and jumping becomes stronger than the forces holding their own atoms together. The molecules break apart into fragments, and the fragments slam into each other hard enough to form new molecules.

That's what I've thought for many years, along with most cooks and confectioners and carbohydrate chemists: heat melts sugar, and then begins to break it apart and create the delicious mixture we call caramel.

And we've all been wrong.

It turns out that, strictly speaking, sugar doesn't actually melt. And it can caramelize while it's still solid. So proved chemist Shelly Schmidt and her colleagues at the University of Illinois in studies published last year.

It's dismaying to think that so many could be so wrong for so long about such a basic ingredient and process! But it's also a rare opportunity to rethink the possibilities of the basic. Here's a plateful of possibilities; scroll down for more.

Professor Schmidt's group made their discovery when they tried to nail down the precise melting point of sucrose. The figures reported in the technical literature vary widely, and it wasn't clear why.

The melting point of a substance is the temperature at which it turns from a solid into a liquid while maintaining its chemical identity. When solid ice turns into liquid water, for example, the molecules of H2O move fast enough to escape the attractive forces of their neighbors, but they're still H2O. And it doesn't matter how fast the substance heats up: the melting point is the same. Ice melts at 32°F/0°C. Always.

After careful analysis, Professor Schmidt found that whenever sugar gets hot enough to turn from a solid into a liquid, some of its molecules are also breaking apart. So sucrose doesn't have a true melting point. Instead it has a range of temperatures in which its molecules are energetic enough to shake loose from their neighbors, and a range in which the molecules jitter themselves apart and form new ones. And these two ranges overlap. Whenever sugar gets hot enough to liquefy, it's also breaking down and turning into caramel. But it starts to break down even before it starts to liquefy. And the more that sugar breaks down while it's still solid, the lower the temperature at which it will liquefy.

When we make caramel standing at the stove, we use high heat to liquefy and then brown the sugar in a few minutes, and the liquefying temperature can be upwards of 380°F/190°C. But Professor Schmidt's group found that when they ramped up the heat slowly, over the course of an hour, so that significant chemical breakdown takes place before the solid structure gives way, the sugar liquefied at 290°F/145°C.

I made the caramelized sugars in these photos by putting crystals and cubes in my gas oven at around 250°F/125°C, shielding them with foil above and below to avoid temperature extremes from the cycling heating element, and leaving them there overnight and longer. In the large sugar crystals, which I got in a Chinese market, it's clear that breakdown and caramelization is fastest in the center. That may be because the center is where impurities get concentrated as the crystals are made, and the impurities then kickstart the breakdown process.

Caramel makers have long known that, as is true in most kinds of cooking, the key to caramelization is the combination of cooking temperature and cooking time. But the the temperatures have typically been very high, the times measured in minutes. Now we know that you can caramelize low and very slow and get something different. Sugar breakdown even occurs at ambient storage temperatures, though it takes months for the discoloration and flavor change to become noticeable. For a manufacturer this is undesirable deterioration. But for a cook in search of interesting ingredients, it could be desirable aging.

In a follow-up to her initial scientific reports, Professor Schmidt wrote in Manufacturing Confectioner that

from a practical point of view, caramelization can be thought of as browning of sucrose by applying heat for a length of time. Thus it may be possible to better control the caramelization reaction by identifying the time-temperature conditions that optimize the production of desirable caramel flavors compounds, while minimizing undesirable ones. Confectionery manufacturers and sugar artisans, armed with this new scientific knowledge, may be able to push their craft in unforeseeable directions.

A few weeks ago Evan Kleiman of KCRW'sGood Foodtweeted me a question from one of her listeners: why did the cooking water for a batch of garden green beans turn pink? Not knowing for sure, I guessed that the color came from early stages of the browning discoloration that develops when many fruits and vegetables are cut or damaged. Mushrooms are especially prone to this, and I've noticed that they turn pink before getting more frankly brown.

Shortly after replying to Evan I happened to pull out a half-dozen fava bean plants from my garden, and I harvested pounds of green pods, old and young. I enjoyed the tender ones whole, tossed in a hot pan with a little oil and salt. They're deliciously different from ordinary green beans, with a flavor that's both meaty and flowery, even perfumed. The older pods I pulled open to collect the beans in their tough seedcoats. I blanched a batch of these beans in boiling water to soften the skins and speed the tedious peeling. And I noticed that the blanching water became pink, especially under incandescent light (below).

This reminded me of how quinces and pears can turn pink and even red when they're slowly poached. They do so because they contain compounds called proanthocyanidins, molecules that don't participate in the usual fruit browning, but that fragment during cooking into anthocyanins, the pigments that color most red fruits and vegetables.

So I checked, and it turns out that the skins of most legume seeds, favas included, are rich in the same proanthocyanidins.

New theory! Colorless proanthocyanidins in bean seedcoats release fragments into the cooking water, and these are what turn the water pink.

I thought I could test the theory by cooking some skins by themselves and adding either citric acid or alkaline baking soda to the cooking water. Anthocyanin pigments are sensitive to pH. Acid usually shades them toward the red, alkali toward the blue. So if the fava seed coats are indeed releasing anthocyanins into the cooking water, I should be able to change the water's color.

Well, the color test didn't turn out the way I expected. The acid cooking water was the same pale pink as the neutral water, and it was the alkaline water that turned a deep, winey red (right). I still haven't figured that out.

But I noticed something else that was easy to figure out, and much more useful than any color change. Whenever I cooked fava beans in alkaline water, more than half of them popped their skins in the pot, no hand-peeling needed! And the rest were easy to peel with a gentle squeeze at one end.

Here's a batch of alkaline-blanched favas fresh out of the pot, before any hand-peeling.

The skin-busting effect of alkaline cooking water makes good sense. Acidity maintains the structure of plant cell walls, and alkalinity breaks it down. That's why beans take forever to soften if you try to cook them in a tomato sauce. So soda in the blanching water weakens the fava seed coats enough that many of them rupture on their own in a couple of minutes at the boil, and the remainder easily break between finger and thumb.

Using soda seems such an obvious idea in retrospect that I'm surprised it's not already standard practice. Many recipes for dried favas do call for baking soda to soften their skins and speed the cooking. There must be other cooks out there who've been blanching fresh favas with soda. I'm now one of them.

So: to ease fava peeling, add about a tablespoon of baking soda to a quart of vigorously boiling water, and throw in the beans. Fish them out as they pop their skins so they don't pick up the soda soapiness, and drop them in a bowl of cold water to rinse. After two or three minutes, scoop the remainder into another bowl of water to cool them down. Peel them by gently squeezing on the thick end of the bean, if necessary nicking the thin end with your fingernails.

And check out the color of the cooking water before it disappears down the drain. It's a sign of the chemical defenses concentrated in the seed coats, and their likely nutritional value for us. That's the next challenge: making fava skins delicious enough to keep and eat with pleasure.

While browsing among the vegetable starts at a nursery in Santa Cruz last year, I came across a flat of sugar beets. I'd never tasted sugar beets before. They're a special variety of Beta vulgaris bred for sugar production, with none of the colorful pigments of vegetable beets that would further complicate the manufacture of pristine white crystals. So I bought some seedlings and planted them. They grew well through nearly a year of benign neglect. Last month I dug them up and tried them out.

The beets I grew were irregular in shape and size, the largest weighing in at about a kilogram, or over 2 pounds trimmed of leaves and small roots. Both raw and roasted they had a mild beet flavor and were very sweet indeed. I used my refractometer, a handy instrument that measures the concentration of dissolved materials in liquids, to get a rough idea of the sugar levels in their juice. Most of the sugar beets ran between 15% and 18% dissolved solids, while store-bought red and chioggia beets were closer to 5%.

I was especially curious to see what a home version of beet sugar would be like. Refined white table sugar is manufactured from beets and from sugarcane by extracting the juices from the raw materials, evaporating off their water, and separating the sucrose sugars from everything else, including a host of other plant chemicals and byproducts of the evaporation process. That separation process involves the use of mineral lime, carbon dioxide, charcoal made from various materials (sometimes animal bone), and centrifuges.

Not for the casual sugar-maker! Instead I figured on making an unrefined sugar, a beet version of the delicious palm and cane jaggeries that come from Asia, or the cane panela (piloncillo, papelón) of Latin America, or North American maple sugar.

But I had my doubts about whether unrefined beet sugar would be anything close to delicious. I'd read that unlike the molasses left over from cane sugar manufacture, beet molasses is fit only for livestock feed. I wondered whether that's because the beet residues are intrinsically unpleasant, or are somehow made so by the particular way beets are handled. Where sugar cane grows above ground and is processed shortly after harvest, beets are dug from the soil, have that distinct earthy odor, and may be stored for months in piles 20 feet high, where they remain alive and can deteriorate. Even fully refined beet sugar sometimes ends up with off flavors.

I ended up making small batches of beet sugar in three different ways. Each time I started by washing the beets, and then trimming and peeling them--two steps not in the industrial flowchart that I hoped would minimize off flavors.

The first time around I ran the beets through a juicer, and got about half the starting weight in juice. Rather than boiling it down over high heat, I gently evaporated it in a gas oven set at 250oF. (Gas ovens are well vented, so evaporation-slowing humidity doesn't build up.) The juice quickly turned an unappealing brown-gray and developed a strong beet odor, probably due to browning enzymes and perhaps also enzymes that generate earthy-smelling volatiles. After a few hours, I had a beety sweet syrup that cooled into a dull-colored paste. It was edible, but not especially nice.

Next I shredded the beets in a food processor, simmered the shreds in three times their weight in water to extract their juices, strained out the shreds, and evaporated the liquid down. This syrup and paste had a pleasantly mild beet aroma, but they were still an unfortunate grayish brown.

Finally I tried precooking the beets to kill the enzymes before I shredded them and extracted the juices. I sliced the beets, rinsed them to remove enzymes from the surfaces, then microwaved the slices in batches so that they reached the boiling point quickly, in a couple of minutes. The syrup developed only a faint beet aroma and a light gray color that soon disappeared into a cool-toned brown.

All of the beet syrups and pastes were mainly sweet, but with an edge of acidity and saltiness from the other plant materials that were extracted and concentrated along with the sugars. When I cooked them down to the point that they turned dark brown, all had the pronounced acidity and bitterness of cane molasses along with its characteristic aroma, which mostly masked any beetiness.

So it is indeed possible to make a good unrefined sugar from a vegetable that can be grown almost anywhere. True, it's not very efficient to do so: evaporating off water burns a lot of energy for the amount of sugar you end up with. (At least you can use the beets efficiently: the greens are essentially the same as chard, and you can squeeze or pan-dry the spent shreds to remove excess moisture, add salt and a little of their own sugar to restore some taste, and then toss with starch or beaten egg to make beet hash browns or latkes).

But it's an interesting process and product. Along the way I learned that in the beet-growing areas of Germany, Zuckerrübensirup is sold as a spread and to flavor pumpernickel bread dough and sauerbraten. Earthy sweetness has its uses.

Digging up a long-neglected corner of my San Francisco garden a few weeks ago, I came across what look like the skeleton bulbs of garlic or some other lily relative, the skins and flesh gone, only the reinforcing cellulose fibers left behind.

They reminded me of a garden find I made one spring about twenty years ago, after I'd grown a couple of crazily prolific purple tomatillo plants. The winter had had its slow way with some stray tomatillos, etching away all but the veins of their papery outer husks and in some cases the flesh within, leaving just the seeds to rattle around. The remnants made an absorbing anatomy lesson, an image of the passing pleasures and durable purpose of fruits.

I've been neglecting this blog as I have my garden, and for many of the same reasons. But I'm starting to work both again. There's plenty to unearth and cultivate and share.

PEPPERED as we are by government warnings about the potential health hazards of eating and drinking just about everything, it was refreshing (and perplexing) to see a widely respected food writer assert recently that “people are unnecessarily afraid of bacteria” in the kitchen.

But what about the harmful microbes that could grow on foods if they were not kept either chilled or hot? “Once your stock is cooked, it’s safe to eat,” Mr. Ruhlman wrote. “If there were bad bacteria in it, you’d have killed them.” After the stock has cooled, simply reheat it, he continued, and “any bacteria that landed there and began to multiply will be dispatched well before the stock hits a simmer.”

Sounds plausible, and Mr. Ruhlman and his family are alive and well. But after checking with an independent expert on food safety, I wouldn’t follow this recipe without slapping a biohazard label on my stockpot.

The Food and Drug Administration sets regulations for commercial food production. These specify that cooked foods should sit out at temperatures from 41 degrees to 135 degrees, the range in which bacteria can grow and multiply, for no more than four hours.

Guidelines for the consumer and home cook, which come from the Department of Agriculture Food Safety and Inspection Service, are even stricter. The current brochure, “Keep Food Safe! Food Safety Basics,” on the U.S.D.A. Web site, says not to leave prepared foods in the bacterial growth zone for longer than two hours. And if it’s a 90-degree summer day, cut the two hours to one.

Mr. Ruhlman’s stock spends days in the bacterial growth zone, and he happily makes it into chicken soup for his children.

I’ll admit to violating the guidelines in my own stock-making, though by a few hours, not days. When I cook a roast for dinner, I use leftover scraps and bones to start the stock, simmer it while I clean up, and take the pot off the heat right before I go to bed. At that point it’s too much trouble to cool the hot stock so it won’t warm up its neighbors in the refrigerator. Instead, I cover the pot, leave it at room temperature and reheat it in the morning, about eight hours later, before straining, cooling and refrigerating it. And my stock hasn’t made me or my family ill, either.

Can I be even more relaxed about my stock-making? Or have Mr. Ruhlman and I just been lucky? For an expert opinion, I sent our recipes to O. Peter Snyder, a food scientist and veteran educator and consultant to the food-service industry, who has at times taken issue with government guidelines he considers unnecessarily conservative.

Dr. Snyder replied in an e-mail: “The process described by Mr. Ruhlman is a very high-risk procedure. It depends totally on reheating the stock before it is used to be sure that it doesn’t make anyone ill or possibly kill them.”

It’s a basic fact that every cook should know: bacteria that cause illness inevitably end up on nearly every ingredient we cook with, and even boiling won’t kill all of them.

Boiling does kill any bacteria active at the time, including E. coli and salmonella. But a number of survivalist species of bacteria are able to form inactive seedlike spores. These dormant spores are commonly found in farmland soils, in dust, on animals and field-grown vegetables and grains. And the spores can survive boiling temperatures.

After a food is cooked and its temperature drops below 130 degrees, these spores germinate and begin to grow, multiply and produce toxins. One such spore-forming bacterium is Clostridium botulinum, which can grow in the oxygen-poor depths of a stockpot, and whose neurotoxin causes botulism.

Once they’ve germinated, bacteria multiply quickly in nourishing stock. They can double their numbers every 90 minutes at room temperature, every 15 minutes at body temperature. A single germinated spore can become 1,000 bacteria in a matter of hours, a billion in a few days.

As Dr. Snyder put it, “After sitting on the stove and growing bacteria for two or three days, Mr. Ruhlman’s stock almost certainly has high levels of infectious Clostridium perfringens cells, or Clostridium botulinum or Bacillus cereus cells and their toxins, or some combination thereof.”

Why has the Ruhlman family survived? Because Mr. Ruhlman boils the stock before he serves it, Dr. Snyder wrote. Any active bacteria are killed by holding the stock for a minute at 150 degrees or above, and botulism toxin is inactivated by 10 minutes at the boil.

But quickly reheating a contaminated stock just up to serving temperature won’t destroy its active bacteria and toxins, and the stock will make people sick.

“If Mr. Ruhlman ever has a cup of his three-day-old stock without thoroughly boiling it first, he will probably only do it once,” Dr. Snyder wrote. “It is irresponsible of any cook to prepare food in a way that actually creates a new and significant hazard, even though the hazard may be eliminated in a later step.”

Safety is one problem with keeping a stock at room temperature. Flavor is another. A reboiled three-day-old stock may be safe to eat, but it is now seasoned with millions to billions of dead bacteria and their inactivated toxins. It’s conceivable that they might add an interesting flavor, but more likely that the bacteria have feasted on the stock’s sugars and savory amino acids, the air has oxidized and staled the fat, and the stock has become less tasty.

I spoke with Mr. Ruhlman about Dr. Snyder’s analysis of his stovetop-stored stock. “I agree that I should have been clearer about the importance of the ‘kill step,’ a good 10 minutes at the boil,” he said. “And certainly to make the freshest, cleanest stock, it’s always best to strain, cool and chill it as rapidly as possible.”

What about my lazy method of letting stock cool overnight, then reboiling and refrigerating it first thing in the morning? Dr. Snyder gave it a pass because it would spend only a few hours below 135 degrees, not enough time for the bacterial spores to germinate, start growing and reach hazardous numbers.

Like meat stocks, all moist cooked foods are susceptible to being recolonized by survivalist bacteria. (Baked goods are generally too dry for bacteria; they’re spoiled by molds.) That’s why we should avoid leaving cooked foods out at room temperature for long when we’re preparing for a party or holiday feast (or enjoying their lazy follow-ups), or having a picnic, or packing lunch boxes for young children, who along with the elderly and ill are more vulnerable. It’s best to keep moist lunch items either cold or hot, surrounded by cold packs or in a thermos.

What are the actual odds of getting sick from casual food handling at home? No one really knows. There are many variables involved, and only a small fraction of illnesses are reported, even to a family doctor, since they’re usually brief. But one unambiguous and heartbreaking story can bring home the value of handling food carefully.

In 2008, a 26-year-old Japanese mother in the Osaka region shared a meal of leftover fried rice with her two children, ages 1 and 2. She had prepared and served the rice the day before and kept it at room temperature.

All three became ill 30 minutes after eating the leftovers, and were hospitalized. Both children lost consciousness, and the youngest died seven hours after the meal. Pathologists later reported in the journal Pediatrics that the rice contained a very common spore-forming bacterium, Bacillus cereus, along with a heat-resistant toxin that the bacterium tends to make on starchy foods, and that can cause vomiting even after being heated to the boil.

It may be true that most cases of food-borne illness aren’t that serious, and that most reported cases can be traced to foods that were contaminated during their production or processing. But it is also true that one simple mistake at home can be fatal.

Even though I know this, I tend to discount specific government guidelines because they seem to change arbitrarily, and they don’t seem workable in real life. This is true of the latest U.S.D.A. numbers. It’s unrealistic to expect home cooks to chill or reheat or discard dishes every two hours during a dinner party, or every hour at a summer barbecue.

Dr. Snyder agreed that official pronouncements on food safety can be inconsistent and self-defeating. “The F.D.A. Food Code is very conservatively written,” he wrote. “Four hours after it’s cooked is plenty fast enough to get food into the refrigerator.” And slow enough to relax and enjoy the meal.

Dr. Snyder added that it’s safest to cool leftovers uncovered and in a mass no thicker than two inches, so they cool through quickly. If they’re still hot, start the cooling on the countertop. When the container is no longer hot to the touch, put it in the refrigerator, and cover it once the food is good and cold.

My own everyday approach to safety is to try to keep cooked foods either hot or cold until I’m ready to serve them, get leftovers in the fridge during the pause before dessert or soon after, and reheat leftovers that need it until they’re boiling or steaming.

This set of habits isn’t dictated by an unnecessary, pleasure-killing fear of microbes. It simply acknowledges their inevitable presence in my kitchen, and the fact that both my food and anyone who eats it will be better off if the care I give it doesn’t end with the cooking.

In this month's Curious Cook column in the New York Times, I write about dealing safely with leftovers.

Last April, the noted food writer Michael Ruhlman suggested on Twitter and in his blog that many home cooks are unnecessarily afraid of bacteria, and that it’s okay to prepare a chicken stock and keep it on the stovetop for days at a time. Michael—Mr. Ruhlman in Times parlance—heard back from many “mystified” readers, and suggested to Pete Wells, the editor of the Dining section, that the Times address the issue of safe stock handling. And Pete suggested it to me.

So the column is in no way aimed at Michael, a fine writer with whom I’ve worked in the past. In fact it was his idea. He wanted to understand a complicated issue better, not simply justify his own perspective. I admire him for that.

It turns out that keeping stock on the stovetop is not such a good idea. For an expert opinion I checked with O. Peter Snyder, a consultant and educator whose writings about food safety are unusually crisp and down to earth. His website includes an extensive list of downloadable guides to handling food. Most of these are aimed at the food industry, but there are also guides for the home cook and for organizers of large group events, where a simple mistake can make many people sick.